704 THE JOURNAL OF GEOLOGY. 



The only other alteration noticed in the diallage is along its 

 edges, where brown and green hornblendes are developed, and in 

 one case where the pyroxene is replaced in part by rosettes of 

 chlorite that polarize in bright blue tints. The very deep pink 

 color of some of the diallage plates may be due to incipient alter- 

 ation, as along with the change in color there is produced a finely 

 fibrous structure. The writer has searched earnestly for indica- 

 tions of enstatite 1 in the rock under consideration, but has 

 failed to discover any, though strongly pleochroic hypersthene 

 is present in large quantity in certain of its phases to be men- 

 tioned later. In one or two specimens of the normal gabbro 

 there is also a little hypersthene, but it is not finely fibrous, and 

 it occurs as very compact plates side by side with equally com- 

 pact and very fresh plates of diallage. 



Much of the pyroxene, as has been said, is in the interstices 

 between the plagioclase and therefore is probably younger than 

 this constituent. It is, however, not in the ophitic areas charac- 

 teristic of diabasic pyroxene, but is usually in narrow stringers 

 between the feldspar grains, and between these and the olivine. 

 In some sections every grain of olivine is thus separated from 

 plagioclase (Fig. i), while in other sections, where this is not 

 the case, the diallage is in too small quantity to serve this pur- 

 pose. Narrow rims of this mineral also exist around magnetite 

 and biotite, and they occur between these two minerals and oliv- 

 ine and a fibrous growth that surrounds them, especially the 

 olivine, in a manner resembling a reaction rim. 



Attempts to isolate the diallage for analysis were not success- 

 ful, as it was found impracticable to free its powder from hyper- 

 sthene and the brown earthy decomposition products of olivine. 



The last mentioned mineral is usually quite fresh, and in large 

 quantity, though in a few specimens it is represented by only an 

 occasional grain in the thin section. Since it was one of the first 

 separations from the magma yielding the rock, it is always present 

 in more or less well defined idiomorphic grains. These are 



x Cf. M. E. Wadsworth: Nos. 787 and 692, pp. 90 and 91. Bull. No. 2 Minn. 

 Geol. Survey. 



